Basics of Meat Science

Basics of Meat Science
Jay B. Wenther, Ph.D. American Association of Meat Processors

Introduction
Muscles are highly specialized tissues that are used to provide structural support, create movement and maintain metabolic processes for complete function of the animal. Muscles are highly organized tissues that supply energy for contraction and transmitting that contraction movement to the skeleton.

Throughout the years, meat has been defined by a number of different authors in a variety of ways. Commonly meat is defined as those animal tissues that are suitable for use as food. This includes all processed and manufactured products that might be prepared from these tissues are included in this definition. Meat is considered a source of high quality proteins. The word protein comes from the Greek word proteios, meaning 'primary' which suggests the importance of proteins to the nutritional well being of humans. The properties of meat make it unique and offer the meat processor the opportunity to manipulate the proteins to provide specific processed meat products.

Muscle Composition
Lean meat, i.e. the muscle tissue contains on average 70-75% water, 19-23% crude protein, 3-2% fat, as well as minerals and saccharides in the quantity of around 1% each.

Moisture
Moisture is the greatest single contributor to muscle weight and is easily lost from meat. The moisture retained by the meat is referred to water holding capacity (WHC). The physical and chemical structure of meat is related to the amount of moisture held within the meat. Water can be classified as bound, immobilized, or free.

Bound water is the water that is directly held by chemical bonds to the meat proteins and represents 4 to 5 precent of the total water found in muscle. It remains tightly bound to the muscle proteins through severe mechanical or other physical force.

Immobilized water is indirectly held by electrically charged reactive groups of meat proteins. For meat processors, the more water that is in the immobilized state relates to a greater ability of the product to retain moisture and thus have increased yeilds. The amount of immobilized water in meat can range from approixmately 35-75%.

Free water is water that is held by muscle membranes and capillary action. Processing of meat, such as grinding or comminution damages the muscle membranes and causes the meat to release this free water. Loss of free water from retail cuts is referred to as drip loss or purge. For meat processors, the objective is to convert free water to the immobilized state.

Protein
Muscle proteins can be divided into categories on the basis of the location in the structure of muscle and of muscle fiber, physicochemical properties (e.g. solubility), and functionality in regard to further processing of meat. Muscle proteins form three large groups of proteins referred to as myofibrillar proteins, sarcoplasmic proteins, and connective tissue or stromal proteins.

Myofibrillar proteins consist primarily of myosin, actin, tropomyosin, m-protein, alpha-actinin, beta-actinin, c-protein, troponin T, I, and C as well as other minor proteins associated with the myofibril, but which are present in very small quantities. Myosin and actin together account for 65% of the total muscle protein; tropomyosin and the troponins each contribute an additional 5% and the remaining 25% is composed of the other regulatory and structural proteins. The other myofibrillar proteins, composed mainly of myosin and actin, are also known as "salt-soluble proteins" due to their ability to be solubilized in solutions of neutral salts.

Sarcoplasmic proteins consist of myoglobin, hemoglobin, cytochrome proteins, and a wide variety of endogenous enzymes. These proteins constitute 30-35% of the total muscle protein. Sarcoplasmic proteins are soluble in solution of low salt concentration, but not in water. Since these proteins have been sometimes extracted with pure water, the name "water-soluble proteins" has become common. Myoglobin is presumably the most important protein of sarcoplasm because it is responsible for meat color, which is associated with product quality. Myoglobin consists of a globular protein portion (globin) and a non protein portion called a heme ring. The heme portion of the pigment plays a special role in meat color determined by the oxidation state of iron within the heme ring.

Stromal proteins, or connective tissue proteins, consist primarily of collagen and elastin. Collagen is the single most abundant protein found in mammalian species, being present in bone, skin, tendons, cartilage, and muscle. Collagen, elastin, and lipo proteins of the cell membrane, including sarcoplasmic reticulum, are among the most important connective tissue proteins in the muscle. In muscle, the connective tissue is composed mainly of the protein collagen and serves as an extracellular support for the fiber.

Fat
Fat is also a major component in muscle/meat. Fat is one of the most variable components of meat. Fats are very high energy compounds and contribute not only calories but flavor, juiciness, and texture to meat. The amount of fat can vary extensively in the animal, as well as in or on a cut of meat. It is said that the difference in flavor between species is due to the different make-up of fats in each species.

Fat is found throughout the carcass or cuts of meat. External fat is found on the surface of the carcass or cut and is also known as backfat. Intermuscle fat is found between muscles within a cut and is referred to as seam fat. Intramuscular fat is the flecks of fat inside of muscle and is known as marbling. Marbling contributes to the flavor and juiciness of a cut of meat. The amount of internal fat, also known as kidney, pelvic, and heart fat, is utilized in equations to determine the yeild grade of beef carcasses.

Lipids or fats are composed of glycerol and fatty acids. The fatty acids are attached to the glycerol molecule. Animal fats are composed of mostly triglycerides and make up a majority of the total lipid within animal fats. Triglycerides means that three fatty acids are attached to the glycerol molecule. One fatty acid is esterified to each hydroxyl (O++) portion of the glycerol molecule. It is possible to have lipids that are monoglycerides (one fatty acid on the glycerol molecule) or diglycerides (2 fatty acids on the glycerol).

Fatty acids are chains of repeating methyl units with an acid (carboxyl) group at one end. Fatty acids are named or classified in two general categories. One is according to the number of carbons in the chain length. There are fatty acids from four carbons in the chain length to twenty-four carbons in the chain length. Fatty acids are also classified according to saturation or unsaturation. The carbon atoms have (in simple terms) four reactive sites. If all four reactive sites are filled with hydrogens or another carbon, it is considered to be saturated. If the reactive sites on the carbon atom do not contain three hydrogens, then it is unsaturated. Unsaturated fatty acids have double bonds between the carbons because of the lack of two hydrogen atoms. Polyunsaturated fatty acids have two or more double bonds in the chain length.

Mineral
Meat from all species contains approximately 1% mineral. The mineral content does not fluctuate very much regardless of other changes in composition throughout the life of the animal, etc. From a nutritional standpoint, the red meats are excellent sources of iron and zinc as well as many other minerals. Remember that myoglobin contains iron as an important part of its structure. Calcium is essential for the contraction of muscles, however, it is found in small amounts. Therefore, meat is not considered a good nutritional source of calcium.

Muscle Structure
Meat can be divided into three distinct muscle types:  Skeletal muscle, smooth muscle, and cardiac muscle. Skeletal muscles constitute the bulk (35-65%) of the carcass weight of meat animals and are organs of the muscular system that are attached directly or indirectly to bones. Smooth muscle is the muscle that makes up the digestive tract. The heart is composed of cardiac muscle.

From an economic standpoint, skeletal muscles are the most important of the three types of muscles. These muscles facilitate movement and/or give support to the body. The smallest independent cellular units of mature skeletal muscle are called fibers. Skeletal muscles are a very complex contractile system made up of cylindrical, multinucleated musclefibers (cells) of varying lengths surrounded by a layer of connective tissue known as the endomysium. Bundles of these muscle fibers are enclosed in a sheath of connectivetissue known as the perimysium, while the entire muscle is surrounded by a denser connective tissue sheath called the epimysium.

The structural complexity of meat can be organized in decreasing size of the functional parts of the muscle as follows; Muscle, muscle bundle, muscle fiber (or cell), myofibril, myofilament. Each muscle fiber contains hundreds of myofibrils.

Myofibrils are linear arrays of cylindrical sarcomeres, which are surrounded on each end by a membrane system that is an elaborate extension of the muscle fiber plasma membrane or sarcolemma. These extensions of the sarcolemma, which are called transverse tubules or t-tubules, enable the sarcolemmal membrane to contact the ends of each myofibril in the muscle fiber.

In between the t-tubules, the sarcomere is covered with a specialized endoplasmic reticulum called the sarcoplasmic reticulum that contains high concentrations of Ca2+. The release of the Ca2+ from the sarcoplasmic reticulum and its interaction within the sarcomeres trigger muscle contraction.

Conversion of Muscle to Meat
During and after slaughter, which is considered post-mortem, many chemical and structural changes take place that will affect the quality and function of meat. These changes are referred to as the conversion of muscle to meat. In the living animal glycogen (animal starch) is transported from the liver to the muscle. When energy is needed for contraction, the glycogen is converted into ATP. For this reaction to occur it is necessary to have oxygen in the system. Remember that myoglobin stores oxygen in the muscle which can be used to create the ATP. After death, the reaction described previously continues as long as there is oxygen in the system. When the oxygen is depleted and cannot be replenished due to the removal of the blood during slaughter, it is not possible to produce additional ATP. When ATP is depleted the thick and thin filaments become locked together and the glycogen is converted to lactic acid. Lactic acid builds up in meat until most all of the glycogen is used up.

After slaughter the muscle develops rigor mortis. The post-mortem changes occur as rigor-mortis develops. When ATP is depleted and the thick and thin filaments of the myofibrils lock together, the muscle is said to be in rigor mortis. Also, during the onset of rigor mortis, the muscles shorten and lock in place. Muscle or meat that is in rigor-mortis is shortened or partially contracted and the thick and thin filaments are loacked which causes the meat to be very tough. Inaddition to the structural changes, the buildup of lactic acid causes the pH of the meat to decrease and become more acidic. The pH of living muscle is around 7, which is neutral in acidity, but after death the pH drops to 5.4-5.7 under normal conditions.

Meat Quality
The drop in pH will influence the overall quality of the final meat product. The pH will affect the water holding capacity of meat and it may affect the color of the meat.

Protein functionality is a general term that has been defined as any physicochemical property that affects the processing and behavior of protein systems as judged by the quality attributes of the final product. The properties of meat constituents that are important for raw materials to encompass that would be untilized through manufacturing phases that will give rise to processed meat products. The major functional properties are: (1) water-binding ability (or water-holding capacity): (2) fat stabilization (or fat emulsification): (3) particle-to-particle binding ability (or protein gelation); and (4) the development of desirable color properties.

Water holding Capacity
When discussing water holding capacity, it is necessary to reiterate the fact that proteins create the predominate mechanism by which water is held with a meat product. Myofibril proteins are the most important proteins in the binding of the water in the meat. These proteins have a net negative charge. The concept or term to be familiar with is isoelectric point (pl). This is the pH where positive charges are equal to negative charges. The isoelectric point of meat is approximately 5.2. As the pH of meat decreases due to the buildup of lactic acid, the pH approaches the isoelectric point. When the pH is 5.2, the difference in positive and negative charges becomes less and less. When positive and negative charges are equal we find the water holding capacity to be at its lowest level.

Fat Emulsification
Meat emulsions are made by grinding or chopping meat and water with the addition of sodium chloride to a fine homogenate, forming the matrix in which animal fat (mostly pork fat) is dispersed. The addition of ingredients plays an important role in the formation and stability of a meat emulsion. A typical procedure begins with the addition of lean meats into the chopper, which contain the highest amounts of myofibrillar proteins. In successful meat emulsions, the salt-soluble, myofibrillar proteins, especially myosin or the actomyosin complex, are generally considered to be the principle emulsifiers. Since these myofibrillar proteins are salt soluble, for the most effective reaction, the salt level should be 4 to 4.5% of the lean meat.

The solubilized (extracted) myosin gives a tacky adhesive body to chopped meat batters. Although lean muscle contains approximately 75% water, additional amounts are usually added to the myosin extraction. Part (approximately half) of the ice or water is added to the extracted myosin. The water is both entrapped in the open myofibrillar structure as well as bound to the negative charges of the protein.

As the temperature increases to 7°C (45°F), the remaining ice or water should be added and allowed to be absorbed by lean tissue. The fat meats and the other ingredients should be added next, while chopping continues to achieve a final emulsion temperature of 13°C (55°F) to 18° (64°F). If all steps have been achieved successfully, the fat will be completely emulsified, meaning that soluble myofibrillar proteins has completely coated each particle of fat and during heat treatment, the protein will denature and bind all the fat.

If a stable emulsion is not acheived there will be evidence of unemulsified fat, unbound moisture or gelatin on the surface or interior of the final cooked product. The unemulsified fat may either be present as fat on the ends of the sausages (fat caps) or a thin coating of grease on the surface of the sausages.

Protein Gelation
Gelation of myofibrillar protein is perhaps the most important property that occurs in restructured, formed, and sausage products and is also responsible for texture, viscoelastic traits, juiciness, and stabilization of fat emulsions in processed products. Application of heat causes a series of events to occur in the myofibrillar proteins used in processed meats. Conformational changes occur during the thermal denaturation of actomysosin.

Protein-protein interaction is a functional event that can be related to structural integrity of meat products through orderly heat-induced aggregations. These aggregations are two-fold, involving the head portion(s) of myosin at temperatures between 30°C and 50°C and the rod segment in the temperature region above 50°C. It is of importance to minimize the pH and muscle variation of the raw meat ingredient and moisture, fat, protein and salt of the meat batter produced.

Color
The color of meat is an important quality attribute. Myoglobin is the pigment or color in meat. Myoglobin is red in color and the concentration of myoglobin in the muscle dictates the color. Myoglobin can exist in three different states: reduced myoglobin, oxymyoglobin, and metmyoglobin. Reduced myoglobin refers to myoglobin that does not have oxygen attached to it. While metmyoglobin contains an iron molecule that has oxidized to a different state (Ferric, Fe++ to Ferrous, Fe+++).

Meat exhibits different colors depending on the presence or absence of oxygen on the myoglobin molecule. Meat after slaughter is oxygen deficient which is to say that the myoglobin molecule has no oxygen attached to it. This is reduced myoglobin and it has a purple color. After cutting the meat the surface is exposed to oxygen in the air and the myoglobin absorbs this oxygen and becomes oxymyoglobin which is a bright red color. The reaction of adding oxygen to the myoglobin is called oxygenation by the chemist.

Packaging films used on fresh meat are designed to be oxygen permeable to help maintain the bright red color. As fresh meat, such as beef, sits in the meat counter it often turns a dark reddish-brown. This color is undesirable to the consumer as they associate this color with old spoiled meat. The color is a result of the myoglobin going through an oxidation process and forming metmyoglobin. The oxidation process causes the iron in the myoglobin molecule to change from the ferric (Fe++) state to the ferrous (Fe+++) state.

Conclusion
The structure and composition of meat will affect the selection of meat for specific purposes. Fresh meat will be selected for different reasons than the meat selected for making processed meat products. Some of the factors considered in selection of meat for specific purposes are the age of the animal at time of harvesting, the amount of connective tissue, the overall quality, the marbling, the color and the lean-to-fat ratio of the raw materials. Overall, knowledge of the basic concepts of meat science and processed meat allows production of high-quality, desirable meat products.

Reference
Romans JR, Jones KW, Costello WJ, Carlson CW, Ziegler PT. 1985. The Meat We Eat. 12th ed. Danville, IL; The Interstate Printers and Publishers, Inc.